This application relates to limited-play optical media with improved shelf-life and playability.
There is a growing trend to utilize optical media for the storage and retrieval of data. This invention relates to machine-readable optical discs of all types, including compact discs, digital videodiscsxe2x80x94or digital versatile discsxe2x80x94(DVD""s) and the like. Conventional optical discs have become widely used for storing digital information including music, video and movies.
In some applications, it is desirable to have a limited life for an optical disc. For example, music and movies on an optical disc are currently rented for a limited time period. When that time has expired, the disc must be returned. A need exists for machine-readable optical discs that do not need to be returned at the end of a rental period. Limited-play discs provide a solution to this problem. Limited play optical discs have been disclosed in U.S. Pat. No. 6,011,772, xe2x80x9cMachine-Readable Optical Disc with Reading-Inhibit Agentxe2x80x9d, issued to SpectraDisc Corporation and in U.S. Pat. No. 5,815,484, xe2x80x9cCopy Protectable, Optical Media Device and Methodology Thereforexe2x80x9d, issued to Hide and Seek Technologies, L.L.C. More recently, SpectraDisc Corp. has disclosed xe2x80x9cMachine-Readable Optical Disc with Reading-Inhibit Agentxe2x80x9d in U.S. Pat. No. 6,343,063 and xe2x80x9cMethods and Apparatus for Rendering an Optically Encoded Medium Unreadablexe2x80x9d in U.S. Pat. No. 6,338,933. U.S. Pat. No. 5,815,484 teaches that limited-play optical discs can be made by superimposing a film containing an oxygen sensitive, reduced form of a dye, including methylene blue, over the surface of the data structures on a disc. Upon exposure to oxygen, the reduced form of the dye, which is essentially colorless, is oxidized to form the deep blue dye, methylene blue which interferes with the ability to read the optical medium.
FIG. 1 shows the layered structure of an non-limited play DVD of the type known in the art. The DVD has first and second outer polycarbonate layers (1, 2), sometimes referred to in the art as L0 and L1 an adhesive layer (3) and an aluminum layer 4. Laser light is introduced through polycarbonate layer L0, (1) and reflects back from the aluminum layer (3) when the disc is played.
The present invention provides a limited-play optical medium with improved shelf stability. The optical medium is formed from a plurality of layers, including in sequence: (a) a first substrate layer; (b) a data layer; (c) a reflective layer; (d) a reactive layer comprising a dye having a reduced state and an oxidized state and further comprising an oxidized form of a reducing agent, said reducing agent being effective to convert the dye from the oxidized state to the reduced state, and (e) a second substrate layer.
The dye in the reduced state is substantially transparent to light of wavelengths used to read the optical medium, and the dye in the oxidized state absorbs light of wavelengths used to read the optical medium. The reflective metal layer is formed from a metal or metal alloy that is not significantly oxidized by the oxidized form of the reducing agent. For example, the reflective metal layer is suitably formed from silver or gold.